In the construction of buildings and installation of forced air heating and air conditioning systems, as well as in other applications such as solar heating arrangements, it is necessary to install and connect together sections of sheet metal ducts to form long, continuous conduits through which fluids, namely hot or cold air, may be conducted. Normally such ducts are rectangular in section, for ease of installation and economy of space. Their end-to-end jointing, of straight sections, curved sections, elbows, corners or the like, poses problems, however. To provide some form of interfitting connecting apparatus considerably increases the expense of manufacture of the sections and particularly installation costs. It is desirable that leakage of air at the joints be minimized. Also, the joints should consist of simple, economical components which can readily and simply used and installed on the duct, either in the shop or on the site. Permanent jointing as by welding is undesirable, since it renders maintenance and replacement of individual sections difficult and is very costly. At the same time the joints must be durable and long lasting, able to withstand the conditions of use (exposure to heated or cooled air flows for periods of many years) in a building for extended periods of time.